Organic electroluminescent device with improved long-term stability

ABSTRACT

An organic electroluminescent device with improved long-term stability is disclosed, comprising either a dual layer of a layer I and a layer II or a mixture of the two layers between an EL multilayer and a second electrode, the layer II being made of at least one alkali metal, alkaline earth metal, or compound thereof, the layer I being formed of at least one material selected from those having chemical formula I and chemical formula II as shown below.

This application is a Divisional of application Ser. No. 09/848,282filed May 4, 2001 now U.S. Pat. No. 6,663,985 which is further aContinuation of 09/178,515 filed Oct. 26, 1998 now U.S. Pat. No.6,248,458 whose disclosure therein is incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a display, and more particularly, to anorganic electroluminescent (EL) device.

2. Background of the Related Art

Organic EL devices, also called organic light emitting diodes (LEDs),are becoming very popular because of their possible application to flatpanel displays (FPDs). Organic EL devices are extremely thin,matrix-addressable and operable at a relatively low voltage, typicallyless than 15 volts. Furthermore, they have additional features suitablefor next generation FPDs such as, among other things, little dependenceon viewing angle and good device-formability on flexible substrates.Organic LEDs differ fundamentally from conventional inorganic LEDs. ForExample, the charge transfer in inorganic LEDs is band-like in natureand the electron-hole recombination results in the interband emission oflight; while organic films are generally characterized by low-mobilityactivated hopping transport, and the emission is excitonic. Furthermore,organic EL devices are substantially different from conventionalinorganic EL devices in other respects, notably in that in that organicEL devices are operable at low DC voltages.

Referring to FIG. 1, a typical organic EL device is shown with a firstelectrode 2 formed on a transparent substrate 1, a hole injecting layer(HIL) 3 and a hole transporting layer (HTL) 4 formed on the firstelectrode 2, a luminescent layer 5 formed on the HTL 4, an electrontransporting layer (ETL) 6 and an electron injecting layer (EIL) 7formed on the luminescent layer 5, and a second electrode 8 formed onthe EIL 7. Any one or more of HIL 3, HTL 4, ETL 6 and EIL 7 may beornitted, depending on the particular device structure adopted.

Electrons and holes injected into the luminescent layer through thesecond electrode 8 and the first electrode 2, respectively, recombine todecay radiatively. For most organic EL devices, the charge injectionbarrier is higher for electrons than for holes. It is well known thatthe electron injection barrier may be lowered by employing a low workfunction material for the second electrode 8. However, low work functionmaterials are chemically reactive, which makes it difficult to use suchmaterials for electrodes. Accordingly, such materials are often used asa second electrode after being alloyed with one of more stablematerials, as seen in the examples of Mg:Ag and Al:Li. However, suchalloyed second electrodes are still less stable, more costly to form,and more difficult to deposit in a uniform film as compared to aluminum.

An even more serious problem often encountered with an alloyed secondelectrode of Mg:Ag or Al:Li is the frequent occurrence of cross talk orcurrent leakage between pixels, which may be attributed to the diffusionof Mg or Li ions across organic layers of the device. This problem canbe greatly alleviated if one selects aluminum as a second electrodematerial. However, in the case of aluminum there is a need to improveits poor electron injecting capability. The electron injectingcapability of a high work function second electrode, such as aluminum,can be significantly enhanced by inserting a very thin layer (typically0.3 nm˜1.0 nm) of an electrically insulating material such as LiF, MgF₂or Li₂O, inserted either between an aluminum electrode and theluminescent layer, or between the aluminum electrode and the ETL(see,for example, IEEE Transactions on Electronic Devices, Vol. 44, No. 8, p1245-1248(1997), the contents of which are incorporated herein in theirentirety).

Li₂O is a particularly interesting material in this regard, in that itis an electrically insulating material with a very low work function.The work function of alkali metals themselves is very low, and itbecomes even lower when oxidized: for example, work function decreasesfrom 2.1 eV for Cs to about 1 eV for Cs₂O. Various alkali metalcompounds have reportedly been used to form an insulating buffer layerfor the purpose of lowering the electron injecting barrier: e.g., Li₂O,LiBO₂, NaCl, KCl, K₂SiO₃, RbCl, and Cs₂O to name a few. Despite thisimprovement, the introduction of the insulating buffer layer poses achallenging new problem, namely, deterioration of adhesion between an ELmultilayer and aluminum, with consequent reduction of life time of thedevice. Experimental results reveal evidence of poor adhesion either atthe buffer layer/aluminum interface or at the EL multilayer/buffer layerinterface. This situation is not unexpected, given the differentcharacteristics of materials involved. In summary, organic EL devices ofthe related art have at least two basic drawbacks, namely, poor adhesionand short life time.

The above references are incorporated by reference herein whereappropriate for appropriate teachings of additional or alternativedetails, features and/or technical background.

SUMMARY OF THE INVENTION

Accordingly, the present invention is directed to an organicelectroluminescent device that substantially obviates one or more of theproblems, limitations and/or disadvantages of the related art.

An object of the present invention is to provide an organic EL devicewith a long life time and have high efficiency.

To achieve these and other advantages, and in accordance with thepresent invention as embodied and broadly described herein, the organicelectroluminescent device comprises at least one organic EL multilayerdisposed between a first electrode and a second electrode, and a layerI, disposed between the organic EL multilayer and the second electrode,including at least one first material from the group consisting ofcompounds of chemical formula I:

where:

each of R₁ to R₄ is independently hydrogen, an alkyl or alkoxy grouphaving from 1 to 5 carbon atoms, aryl, aryloxy or a halogen, or at leastone among pairs of adjacent substituents of R₁ through R₄ may form anfive or six-numbered conjugated cyclic ring which may includes carbon,nitrogen, or sulfur; and,

A each independently denotes hydrogen, an alkyl group having from 1 to 5carbon atoms, or aryl.

Herein, the term the organic EL multilayer may encompass a plurality oflayers comprising a luminescent layer and typically one or more of theHIL, HTL, ETL, and EIL.

In a preferred form of the invention, said each of R₁ to R₄ isindependently hydrogen, an alkyl and alkoxy group having from 1 to 5carbon atoms, phenoxy, phenyl, naphtyl, fluorine, chlorine, or bromine,and said A each independently denotes methyl, ethyl, phenyl, orhydrogen.

In a highly preferred form of the invention, said R₁ is an alkyl andalkoxy group having from 1 to 5 carbon atoms, phenoxy, or phenyl, andsaid each of R₂ to R₄ and A is independently hydrogen.

In addition, the above-mentioned organic electroluminescent device mayfurther comprises a layer II including at least one second materialselected from the group consisting of an alkali metal, an alkaline earthmetal, and a compound thereof, which may be disposed between the layer Iand the second electrode. Preferably, said second electrode may comprisealuminum. Also, said layer II may comprise Li₂O. Said layer I may have athickness of from about 0.5 nm to about 50 nm, and said layer II mayhave a thickness of from about 0.2 nm to about 3 nm.

According to another aspect of this invention, there is provided a meansto improve the life time bf an organic EL device, as well as theelectron injecting capability, by inserting, instead of a dual layer ofthe layer I and the layer II, a mixed layer comprising a mixture of thecomponents of layer I and layer II, wherein the mixed layer is insertedbetween the organic EL multilayer and the second electrode. The mixedlayer is formed by the co-deposition of (1) at least one first materialselected from the group consisting of compounds of chemical formula Iand (2) at least one second material selected from the group consistingof an alkali metal, an alkaline earth metal, and a compound thereof Theratio between the first and second materials in the mixed layer can beeither fixed or varied as a function of position, i.e., by forming aconcentration gradient of the first and second material within the mixedlayer. Preferably, said mixed layer may have a thickness of from about0.5 nm to about 10 nm. Also, said second electrode comprises aluminum.Said second material may comprise Li₂O.

The present invention is also directed to an organic electroluminescent(EL) device comprising: at least one organic EL multilayer between afirst electrode and a second electrode, and a layer I, disposed betweenthe organic EL multilayer and the second electrode, including at leastone first material selected from porphyrinic compounds. Herein, the termthe organic EL multilayer may encompass a plurality of layers comprisinga luminescent layer and typically one or more of the HIL, HTL, ETL, andEIL. The layer I includes at least one porphyrinic compound and servesprincipally to improve adhesion between the organic EL multilayer andthe second electrode, while retaining good electron transportingcapability.

Preferred porphyrinic compound according to the invention are those ofthe structural formula II as shown below.

where:

A each independently denotes —N═ or —C(R)═, and R is hydrogen, alkyl,alkoxy, aralkyl, alkaryl, aryl, or a heterocyclic group;

M comprises an element selected from groups IA, IIA, IIIA and IVA, andthe third, fourth, fifth and sixth periods of the periodic table;

Y is alkoxy, phenoxy, alkylarnino, arylarnino, an alkylphosphinic group,an arylphosphinic group, alkylsulfur or arylsulfur, or an elementselected from groups VIA and VIIA of the periodic table;

n is an integer of 0, 1, or 2; and,

B₁ through B₈ each independently represents hydrogen, alkyl, aryl,alkoxy, aryloxyalkyl, hydroxy, hydroxyalkyl, aralkyl, alkylamino,arylamino, alkylthiol, arylthiol, nitroalkyl, alkylcarbonyl,alkoxycarbonyl, phenyl, amino, cyanyl, naphthyl, alkaryl, a halogen or aheterocyclic group, or at least one among pairs of adjacent substituentsof B₁ through B₈ may form an unsaturated or saturated five, six, orseven-numbered ring which may include substituents such as alkyl, aryl,alkoxy, aryloxyalkyl, hydroxy, hydroxyalkyl, aralkyl, alkylamino,arylamino, nitroalkyl, alkylcarbonyl, alkoxycarbonyl, phenyl, amino,cyanyl, naphthyl, alkaryl, a halogen or a heterocyclic group. Preferredfive, six or seven-numbered rings are those which include carbon,sulfur, oxygen or nitrogen ring atoms.

More highly preferred examples of useful porphyrinic compounds arephtalocyanines. Exemplary preferred materials are those of structuralformulas III and IV as shown below:

where:

A each independently denotes —N═ or —C(R)═; and R is hydrogen, alkyl,alkoxy, aralkyl, alkaryl, aryl, or a heterocyclic group;

M comprises an element selected from groups IA, IIA, IIIA and IVA, andthe third, fourth, fifth and sixth periods of the periodic table;

Y is alkoxy, phenoxy, alkylamino, arylamino, an alkylphosphinic group,an arylphosphinic group, alkylsulfur, or arylsulfur, or an elementselected from groups VIA and VIIA of the periodic table;

n is an integer of 0, 1, or 2; and,

X₁ through X₈ each independently represent hydrogen, alkyl, aryl,alkoxy, aryloxyalkyl, hydroxy, hydroxyalkyl, aralkyl, alkylamino,arylamino, alkylthiol, arylthiol, nitroalkyl, alkylcarbonyl,alkoxycarbonyl, phenyl, amino, cyanyl, naphthyl, alkaryl, a halogen or aheterocyclic group, or at least one among pairs of adjacent substituentsof X₁ through X₈ may form an unsaturated or saturated five, six, orseven-numbered ring which may include substituents such as alkyl, aryl,alkoxy, aryloxyalkyl, hydroxy, hydroxyalkyl, aralkyl, akylamino,arylamino, nitroalkyl, alkylcarbonyl, alkoxycarbonyl, phenyl, amino,cyanyl, naphthyl, alkaryl, a halogen or a heterocyclic group. Preferredfive, six or seven-numbered rings are those which include carbon,sulfur, oxygen or nitrogen ring atoms.

The most preferred examples of phthalocyanines are those of structuralformulas V and VI as shown below:

where:

M is one of Co, AlCl, Cu, 2Li, Fe, Pb, Mg, SiCl₂, 2Na, Sn, Zn, Ni, Mn,VO, 2Ag, MnCl, SnCl₂, and TiO.

In addition, the above-mentioned organic electroluminescent device mayfurther comprise a layer II including at least one second materialselected from the group consisting of an alkali metal, an alkaline earthmetal, and a compound thereof.

According to another aspect of this invention, there is provided a meansto improve the life time of an organic EL device, as well as theelectron injecting capability, by inserting, instead of a dual layer ofthe layer I and the layer II, a mixed layer comprising a mixture of thecomponents of layer I and layer II, wherein the mixed layer is insertedbetween the organic EL multilayer and the second electrode. The mixedlayer is formed by the co-deposition of (1) at least one first materialselected from the group consisting of compounds of chemical formula IIand (2) at least one second material selected from the group consistingof an alkali metal, an alkaline earth metal, and a compound thereof Theratio of the first and second materials in the mixed layer can be eitherfixed or varied as a function of position, i.e., by forming aconcentration gradient of the first and second materials within themixed layer.

Additional advantages, objects, and features of the invention will beset forth in part in the description which follows and in part willbecome apparent to those having ordinary skill in the art uponexamination of the following or may be learned from the practice of theinvention. The objects and advantages of the invention may be realizedand attained as particularly pointed out in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in detail with reference to thefollowing drawings in which like reference numerals refer to likeelements wherein:

FIG. 1 illustrates a cross-sectional view of a related art organic ELdevice; and,

FIG. 2 illustrates a cross-sectional view of an organic EL device inaccordance with a preferred embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 2, the organic EL device in accordance with apreferred embodiment of the present invention includes a laminatedstructure of a transparent substrate 21, a first electrode 22, anorganic EL multilayer 23 (which may comprise a HIL, a HTL, a luminescentlayer, and an ETL), a second electrode 24, and a protection film 25. Inaddition, disposed between the organic EL multilayer 23 and the secondelectrode 24 are a layer I 26 and a layer II 27 which primarily servefor improving inter-layer adhesion and electron injection, respectively.The present invention may include only the layer I 26, or include bothof the layer I 26 and the layer II 27. In a preferred embodiment of theinvention, layer I comprises at least one first material from the groupconsisting of compounds of chemical formula I:

where:

each of R₁ to R₄ is independently hydrogen, an alkyl or alkoxy grouphaving from 1 to 5 carbon atoms, aryl, aryloxy or a halogen, or at leastone among pairs of adjacent substituents of R₁ through R₄ may form afive or six- numbered conjugated cyclic ring which may includes carbon,nitrogen, or sulfur; and,

A each independently denotes hydrogen, an alkyl group having from 1 to 5carbon atoms, or aryl.

Herein, the term the organic EL multilayer may encompass a plurality oflayers comprising a luminescent layer and typically one or more of theHIL, HTL, ETL, and EIL.

Preferably, said each of R₁ to R₄ of chemical formula I is independentlyhydrogen, an alkyl and alkoxy group having from 1 to 5 carbon atoms,phenoxy, phenyl, naphthyl, fluorine, chlorine, or bromine, and said Aeach independently denotes methyl, ethyl, phenyl, or hydrogen.

In a highly preferred form of the invention, said R₁ is an alkyl andalkoxy group having from 1 to 5 carbon atoms, phenoxy, or phenyl, andsaid each of R₂ to R₄ and A is independently hydrogen.

In addition, the above-mentioned organic electroluminescent device mayfurther comprise a layer II, which may be disposed between the layer Iand the second electrode. The layer II 27 is designed to improve theelectron injection capability and comprises at least one materialselected from the group consisting of an alkali metal, an alkaline earthmetal, and a compound thereof. Exemplary preferred materials comprisinglayer II 27 include Li, Cs, Li₂O, and LiF. The second electrode 24 isformed of a metal or its alloy, most preferably aluminum.

The organic EL device of the present invention having the layer I 26 andthe layer II 27 thus stacked between the organic EL multilayer 23 andthe second electrode 24 shows a substantially prolonged life time aswell as a high luminance. The layer I 26 and the layer II 27 may bestacked in the opposite sequence to that shown in FIG. 2, which resultsin decreased luminance and life time. The layer I 26 and the layer I 27may have a thickness of from about 0.5 nm to about 50 nm and from about0.2 nm to about 3 nm, respectively.

According to another aspect of this invention, there is provided a meansto improve the life time of an organic EL device, as well as theelectron injecting capability, by inserting, instead of a dual layer ofthe layer I and the layer II, a mixed layer comprising a mixture of thecomponents of layer I and layer II, wherein the mixed layer is insertedbetween the organic EL multilayer and the second electrode. The mixedlayer is formed by the co-deposition of (1) at least one first materialselected from the group consisting of compounds of chemical formula Iand (2) at least one second material selected from the group consistingof an alkali metal, an alkaline earth metal and a compound thereof Theratio of the first and second materials in the mixed layer can be eitherfixed or varied as a function of position, i.e., by forming aconcentration gradient of the first and second materials within themixed layer.

Preferably, the mixed layer may have a thickness of from about 0.5 nm toabout 10 nm.

Exemplary preferred materials comprising layer II 27 include Li, Cs,Li₂O, and LiF. The second electrode 24 is formed of a metal or itsalloy, most preferably aluminum.

In addition, the layer I may comprises at least one porphyrinic compoundand serves principally to improve the adhesion between the organic ELmultilayer 23 and the second electrode 24, while retaining good electrontransporting capability. Preferred porphyrinic compounds are those ofstructural formula II as shown below:

where:

A each independently denotes —N═ or —C(R)═; and R is hydrogen, alkyl,alkoxy, aralkyl, alkaryl, aryl, or a heterocyclic group;

M comprises an element selected from groups IA, IIA, IIIA and IVA, andthe third, fourth, fifth and sixth periods of the periodic table;

Y is alkoxy, phenoxy, alkylamino, arylamino, an alkylphosphinic group,an arylphosphnic group, alkylsulfur or arylsulfur, or an elementselected from groups VIA and VIIA of the periodic table;

n is an integer of 0, 1, or 2; and,

B₁ through B₈ each independently represent hydrogen, alkyl, aryl,alkoxy, aryloxyalkyl, hydroxy, hydroxyalkyl, aralkyl, alkylamino,arylamino, alkylthiol, arylthiol, nitroalkyl, alkylcarbonyl,alkoxycarbonyl, phenyl, amino, cyanyl, naphthyl, alkaryl, ahalogenoraheterocyclic group, or at least one among pairs of adjacentsubstituents of B1 through B8 may form an unsaturated or saturated five,six, or seven-numbered ring which may include substituents such asalkyl, aryl, alkoxy, aryloxyalkyl, hydroxy, hydroxyalkyl, aralkyl,alkylamino, arylamino, nitroalkyl, alkylcarbonyl, alkoxycarbonyl,phenyl, amino, cyanyl, naphthyl, alkaryl, a halogen, or a heterocyclicgroup. Preferred five, six, or seven-numbered rings are those whichinclude carbon, sulfur, oxygen and nitrogen ring atoms.

More highly preferred examples of useful porphyrinic compounds arephthalocyanines. Exemplary preferred phthalocyanines are those ofstructural formulas III and IV as shown below:

where:

A each independently denotes —N═ or —C(R)═; and R is hydrogen, alkyl,alkoxy, aralkyl, alkaryl, aryl, or a heterocyclic group;

M comprises an element selected from groups IA, IIA, IIIA and IVA, andthe third, fourth, fifth and sixth periods of the periodic table;

Y is alkoxy, phenoxy, alkylamino, arylarmino, an alkylphosphinic group,an arylphosphinic group, alkylsulfur, or arylsulfur or an elementselected from groups VIA and VIIA of the periodic table;

n is an integer of 0, 1, or 2; and,

X₁ through X₈ each independently represent hydrogen, alkyl, aryl,alkoxy, aryloxyalkyl, hydroxy, hydroxyalkyl, aralkyl, alkylamino,arylainino, alkylthiol, arylthiol, nitroalkyl, alkylcarbonyl,alkoxycarbonyl, phenyl, amino, cyanyl, naphthyl, alkaryl, a halogen, ora heterocyclic group, or at least one among pairs of adjacentsubstituents of X1 through X8 may form an unsaturated or saturated five,six, or seven-numbered ring, which may include substituents such asalkyl, aryl, alkoxy, aryloxyalkyl, hydroxy, hydroxyalkyl, aralkyl,alkylamino, arylamino, nitroalkyl, alkylcarbonyl, alkoxycarbonyl,phenyl, amino, cyanyl, naphthyl, alkaryl, a halogen, or a heterocyclicgroup. Preferred five, six, or seven-numbered rings are those whichinclude carbon, sulfur, oxygen or nitrogen ring atoms.

Preferably, M of chemical formulas II˜IV is selected from the groupconsisting of 2Li, 2Na, Mg, Ti, V, Cr, Mn, Fe, Co, Ni, Pt, Cu, 2Ag, Zn,Al, Ga, In, Si, Sn, Pb, 2H, and TiO; and Y of chemical formulas II˜IV isselected from the group consisting of oxygen, fluorine, chlorine,bromine, an alkoxy group (having alkyl moieties containing from about 1to 10 carbon atoms), and a phenoxy group.

The most preferred examples of phthalocyanines are those of structuralformulas V and VI as shown below:

where:

M is selected from the group consisting of Co, AlCl, Cu, 2Li, Fe, Pb,Mg, SiCl₂, 2Na, Sn, Zn, Ni, Mn, VO, 2Ag, MnCl, SnCl₂, and TiO.

The layer II 27 is designed to improve the electron injection capabilityand comprises at least one material selected from the group consistingof an alkali metal, an alkaline earth metal, and a compound thereof.Exemplary preferred materials comprising layer II 27 include Li, Cs,Li₂O, and LiF. The second electrode 24 is formed of a metal or itsalloy, most preferably aluminum.

The organic EL device of the present invention having the layer I 26 andthe layer II 27 thus stacked between the organic EL multilayer 23 andthe second electrode 24 shows a substantially prolonged life time aswell as a high luminance. The layer I 26 and the layer II 27 may bestacked in the opposite sequence to that shown in FIG. 2, which resultsin decreased luminance and life time. The layer I 26 and the layer II 27may have a thickness of from about 0.5 nm to about 50 nm and from about0.2 nm to about 3 nm, respectively.

Exemplary devices have been fabricated in accordance with the presentinvention, and their performance has been compared with that ofbackground art devices: the two devices (A and B) are of the relatedart, and the other two devices (C and D) represent two embodiments ofthe present invention. The basic structure of a related art organic ELdevice for green emission comprises: (1) a first electrode of indium tinoxide (ITO) 150 nm thick, (2) a buffer layer of copper phthalocyanine(CuPc) typically 10 nm˜20 nm thick, (3) a hole transporting layer ofN,N′-diphenyl-N,N-bis(3-methylphenyl)-(1,1′-biphenyl)4,4′-diamine (TPD)typically 30 nm˜50 nm thick, and (4) an emitting layer oftris(8-hydroxy-quinolate)aluminum (Alq₃) 40 nm˜60 nm thick. The device Ahas a second electrode of aluminum formed directly on the organic ELmultilayer (device structure: ITO/CuPc/TPD/Alq₃/Al); while the device Bhas a layer of Li₂O 1 nm thick between the organic EL multilayer and thesecond electrode (device structure: ITO/CuPc/TPD/Alq₃/Li₂O (1 nm)/Al).The device C has a structure of ITO/CuPc/TPD/Alq₃/CuPc(2 nm)/Li₂O(1nm)/Al, while the device D has a structure of ITO/CuPc/TPD/Alq₃/Li₂O(1nm)/CuPc(2 nm)/Al. In short, the layer-forrning sequence of Li₂O(1 nm)and CuPc(2 nm) is reversed for the devices C and D. Finally, all deviceshave been encapsulated in an inert atmosphere.

TABLE I lists the voltage measured between a first electrode and asecond electrode, luminance, and life time of the device, with eachsample device subjected to a constant current density of 3 mA/cm².

TABLE I voltage (V) luminance (cd/m²) life time Device A 6 50 short(less than 1 hour) Device B 6 100 short (less than 1 hour) Device C 6150 long (longer than 2000 hours) Device D 8 130 intermediate (roughly100 hours)

The life time quoted in TABLE I represents the time by which luminancedrops to half the initial value for each device. It is apparent fromTABLE 1 that the devices C and D of the present invention have superiorlong-term stability as compared with the devices A and B of the relatedart. This result may be explained as follows: Li₂O shows poor adhesionto both Alq₃ and aluminum (e.g., device B). In addition, a 1 nm-thicklayer of Li₂O does not form a complete and uniform layer, but rather atype of island structure. As a result, in device C, a part of the CuPclayer is apparently in direct contact with aluminum through voids in theLi₂O layer, which significantly contributes to enhancing the adhesion ofthe organic/metal interface and consequently the life time of thedevice, such as device C. The copper ion in CuPc is probably responsiblefor the relatively strong bonding between CuPc and aluminum. Compared todevice C, device D exhibits a higher driving voltage because of theraised electron-injection barrier, due to the presence of CuPc betweenLi₂O and aluminum, and consequently device D exhibits a shorter lifetime due to the increased electrical and thermal stress.

According to another embodiments of the invention, there is provided ameans to improve the life time as well as the electron injectingcapability of an organic EL device, by inserting, instead of a duallayer of the layer I 26 and the layer I 27 in FIG. 2, a mixed layercomprising layer I 26 and layer II 27 between the organic EL multilayerand the second electrode. The mixed layer is formed by the co-depositionof (1) a first material comprising at least one porphyrinic compound;and (2) a second material comprising at least one of an alkali metal, analkaline earth metal, and a compound of thereof The use of the mixedlayer improves adhesion, but deteriorates electron injection slightly.The mixing ratio between the two groups of materials can be fixedthroughout the layer, or varied as a function of position (i.e., to forma concentration gradient of the components within the layer). Accordingto a preferred embodiment, the mixed layer has a concentration gradientwherein: (1) the relative concentration of the second materialcomprising at least one of an alkali metal, an alkaline earth metal, andcompounds thereof is zero at the interface with the EL multilayer, andunity at the interface with the second electrode, and varying graduallyin between; and (2) the relative concentration of the first materialcomprising porphyrinic compounds in unity at the interface with the ELmultilayer, and zero at the interface with the second electrode, andvarying gradually in between. The thickness of the mixed layer ispreferably in the range of from about 0.5 nm to about 10 nm.

Advantages of organic EL devices of the present invention include thefollowing. The insertion of either a dual layer of layer I 26 and layerII 27, or a mixture of the two layers between the organic EL multilayerand the second electrode leads to significant improvements in thelong-term stability as well as the luminance of an organic EL devicewherein layer I 26 comprises at least one compound selected from thegroup consisting of those having chemical formula I and II, and layer II27 comprises at least one of an alkali metal, an alkaline earth metal,and a compound thereof.

The foregoing embodiments are merely exemplary and are not to beconstrued as limiting the present invention. The present teaching can beeasily applied to other types of apparatuses. The description of thepresent invention is intended to be illustrative, and not to limit thescope of the claims. Many alternatives, modifications, and variationswill be apparent to those skilled in the art. In the claims,means-plus-function clauses are intended to cover the structuresdescribed herein as performing the recited function and not onlystructural equivalents but also equivalent structures.

1. An organic electroluminescent (EL) device comprising: at least oneorganic EL multi-layer comprising a luminescent layer and one or more ofa hole injecting layer, a hole transporting layer, an electrontransporting layer or an electron injecting layer between a firstelectrode and a second electrode, and a layer I positioned between saidsecond electrode and said at least one organic EL multi-layer, saidlayer I comprising at least one first material selected from porphyriniccompounds and further comprising a layer II including at least onesecond material selected from the group consisting of an alkali metal,an alkaline earth metal, and a compound thereof wherein said layer II ispositioned between said layer I and said organic EL multi-layer or saidlayer II is positioned between said second electrode and said layer I.2. The device as claimed in claim 1, wherein said at least one firstmaterial has the following structure:

where: A each independently denotes —N═ or —C(R)═, and R is hydrogen,alkyl, alkoxy, aralkyl, alkaryl, aryl, or a heterocyclic group; Mcomprises an element selected from groups IA, IIA, IIIA and IVA, and thethird, fourth, fifth and sixth periods of the periodic table; Y isalkoxy, phenoxy, alkylamino, arylamino, an alkylphosphinic group, anarylphosphinic group, alkylsulfur or arylsulfur, or an element selectedfrom groups VIA and VIIA of the periodic table; n is an integer of 0, 1,or 2; and B₁ through B₈ each independently represent hydrogen, alkyl,aryl, alkoxy, aryloxyalkyl, hydroxy, hydroxyalkyl, aralkyl, alkylamino,arylamino, alkyithiol, arylthiol, nitroalkyl, alkylcarbonyl,alkoxycarbonyl, phenyl, amino, cyano, naphthyl, alkaryl, a halogen or aheterocyclic group, or at least one among pairs of adjacent substituentsof B₁, through B₈ form an unsaturated or saturated five, six, orseven-numbered ring.
 3. The device as claimed in claim 1, wherein saidat least one first material comprises a compound selected from the grouprepresented by chemical formulas III and IV as shown below:

where: A each independendy denotes —N═ or —C(R)═; and R is hydrogen,alkyl, alkoxy, aralkyl, alkaryl, aryl, or a heterocyclic group; Mcomprises an element selected from groups IA, IIA, IIIA and IVA, and thethird, fourth, fifth and sixth periods of the periodic table; Y isalkoxy, phenoxy, alkylamino, arylamino, an alkylphosphinic group, anarylphosphinic group, alkylsulfur or arylsulfur, or an element selectedfrom groups VIA and VIIA of the periodic table; n is an integer of 0, 1,or 2; and, X₁, through X₈ each independently represent hydrogen, alkyl,aryl, alkoxy, aryloxyalkyl, hydroxy, hydroxyalkyl, aralkyl, alkylamino,arylamino, alkylthiol, arylthiol, nitroalkyl, alkylcarbonyl,alkoxycarbonyl, phenyl, amino, cyano, naphthyl, alkaryl, a halogen or aheterocycic group, or at least one among pairs of adjacent substituentsof X₁, through X₈ form an unsaturated or saturated five, six, orseven-numbered ring.
 4. The device as claimed in claim 1, wherein saidat least one first material comprises a compound selected from the grouprepresented by chemical formulas V and VI as shown below:

where: M is one of Co, AlCl, Cu, 2Li, Fe, Pb, Mg, SiCl₂, 2Na, Sn, Zn,Ni, Mn, VO, 2Ag, MnCl, SnCl₂, and TiO
 5. The device as claimed in claim1, wherein said layer II comprises Li₂O.
 6. The device as claimed inclaim 1, wherein said layer I has a thickness of from about 0.5 nm toabout 50 nm and said layer II has a thickness of from about 0.2 nm toabout 3 nm.